Section-specific H+ fluxes in renal tubules of fasted and fed goldfish
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abstract
A recent study demonstrated that in response to a feeding-induced metabolic acidosis, goldfish Carassius auratus adjust epithelial protein and/or mRNA expression in their kidney tubules for multiple transporters known to be relevant for acid-base regulation. These include Na+/H+-exchanger, V-H+-ATPase, cytoplasmic carbonic anhydrase, HCO3−-transporters, and Rhesus proteins. Consequently, renal acid output in the form of protons and NH4+ increases. Little, however, is known about mechanistic details of renal acid-base regulation in C. auratus and teleost fishes in general. The present study applied the Scanning Ion-selective Electrode Technique (SIET) to measure proton flux in proximal, distal and connecting tubules of goldfish. We detected increased H+ efflux into the extracellular fluid from the tubule in fed animals, resulting from paracellular back-flux of H+ through the tight junction. By applying inhibitors for selected acid-base regulatory epithelial transporters, we found that cytosolic carbonic anhydrase and HCO3− transporters were important in mediating H+ flux in all three tubule segments of fed goldfish. Contrastingly, V-H+-ATPase seemed to play a role for H+ flux only in proximal and distal tubule, and Na+/H+-exchanger in proximal and connecting tubule. We develop working models for transport of acid-base relevant equivalents (H+, HCO3−, NH3/NH4+) for each tubule segment in C. auratus kidney. While the proximal tubule appears to play a major role in both H+ secretion and HCO3− reabsorption, the distal and connecting tubules seem to mainly serve for HCO3− reabsorption and NH3/NH4+ secretion.
